The altered compositions of different polymeric materials provide them with significantly different properties. Modifying a particular molecular composition may allow for the production of a material having specific properties required for a particular use.
Recent work has investigated the possibility of producing a thermoplastic material with elastomeric properties. Elastomers generally take the form of thermoset, as opposed to thermoplastic, polymers. For example, natural and man-made rubbers undergo the necessary step of vulcanization in their production. The resultant cross-linked mass cannot melt under the influence of heat. Rather, it merely burns. Silicone rubbers, having a polysiloxane structure, also undergo cross-linking to produce their elastomeric properties. Again, they do not melt when heated.
In their series of U.S. Pat. Nos. 3,485,787, 3,830,767, 4,006,116, 4,039,629, and 4,041,103, the Shell Oil Company has attempted to provide an elastomer with thermoplastic properties through the use of block copolymer formulations. Typically, a block copolymer has a formula of EQU A--B--A (1)
In the thermoplastic elastomers, the middle block B of the copolymer posesses the desired elastomeric properties.
However, unlike the commonly used rubber materials, the elastomeric middle blocks do not cross-link throughout the material to provide an integral mass. Rather, the terminal blocks A provide the requisite cohesion between the macromolecules. These terminal blocks apparently bond together in the usual thermoplastic fashion. In Shell's material, the terminal thermoplastic blocks represent a minor portion of the macromolecules' total weight. Accordingly, they adhere to each other in the form of relatively small particles embedded within the elastomeric mass of the middle B blocks.
For the terminal blocks, Shell typically employs polymers formed from monoalkenyl arenes, of which styrene represents a typical example.
For the middle block B, Shell uses a conjugated diene polymer. Butadiene and isoprene provide examples of the elastomeric middle blocks.
The copolymer may contain more than three blocks suggested by formula (1) above. Thus it may have several interspersed A and B blocks linearly interconnected as EQU A--B--A--B--A--B. (2)
Alternately or additionally, the block copolymer may have blocks with a branched connection to the main chain as ##STR1## Or, it may have a simple A--B structure.
In the ensuing discussion, the simple formula (1) will encompass all of these variations with B representing the elastomeric block. The letter A will designate the thermoplastic binding block which constitutes the terminal blocks on a three-block molecule. The block copolymers may include various other ingredients such as mineral oil, polystyrene, polypropylene and antioxidants.
In their U.S. Pat. No. 3,485,787, assigned to Shell, W. R. Haefele et al. incorporate mineral oil to extend the block copolymer. In this particular patent, the middle elastomeric block undergoes hydrogenation to produce an ethylene-butylene block from a butadiene block. To prevent bleeding of the extending oil, N. J. Condon, in U.S. Pat. No. 3,830,767, incorporates a petroleum hydrocarbon wax into the block copolymer.
R. J. G. Dominguez, in Shell's U.S. Pat. No. 4,006,116, and G. R. Heimes et al., in U.S. Pat. No. 4,039,629, create blends of styrene-ethylene-butylene-styrene block copolymers having different molecular weights for both the styrene end blocks and the ethylene-butylene middle blocks. These blends apparently produce a product having superior characteristics for their intended footwear use.
S. Davidson et al., in their U.S. Pat. No. 4,041,103, blend a styrene-ethylene-butylene-styrene block copolymer with a polyamide polymer. The patent states that the product achieves improved dimensional stability at high temperatures.
Notwithstanding the various efforts to produce a high quality thermoplastic elastomer, the resulting materials do not display many of the qualities associated with the usual rubber materials. For example, when extruded the thermoplastic elastomers do not have the surface smoothness nor the elasticity of the natural, manmade, or silicone rubbers.
A particular need for an improved thermoplastic material, especially with elastomeric properties, appears in the medical field. Notwithstanding the array of available plastics, the problems of toxicity, incompatibility, surface roughness, lack of flexibility, and others have not submitted to a completely satisfactory solution.
D. H. Kaelble, in U.S. Pat. No. 4,123,409, provides a thermoplastic elastomer primarily for sealing a stoma passing through human tissue. The material utilizes a block copolymer having thermoplastic terminal blocks and an elastomeric intermediate block. The copolymer receives an equal amount of high molecular-weight oil compatible and associatable with the elastomeric block of the copolymer. Thus, a copolymer having a hydrocarbon elastomeric block will incorporate a hydrocarbon mineral oil in order to achieve its desired improvement. Similarly, where a polysiloxane represents the elastomeric block, a silicone oil forms part of the end product. The resulting materials, according to the patent, display an increased pliancy as well as an ability to wet skin.
R. K. Bernstein et al, in their U.S. Pat. No. 3,034,509, add about 0.15 to 1.00 percent by weight of a silicone oil to polyethylene used in surgical tubing. The addition of the silicone oil reduces the blood's toxic reaction to the tubing. It also helps retard blood coagulation where the tubing contacts living tissue for extensive periods of time.
The endotracheal tube represents a further medical device which also receives a substantial investment of time and effort to improving it. The problems associated with an endotracheal tube receive a discussion of W. Wu et al., Critical Care Medicine, 1, 197(1973), U. Nordin, Acta Otorlaryngol Suppl., 345, 7(1975), and W. N. Bernhard et al., Anesthesiology, 48, 413(1978). These articles focus on the inflatable cuff surrounding the main shaft of the endotracheal tube. The cuff securely lodges the endotracheal tube within the patient's trachea at the location determined by the anesthesiologist. Further, the cuff should completely fill at least a portion of the trachea to prevent aspiration fluids, such as saliva, from passing through the trachea to the patient's lungs.
Two types of cuffs currently find use on endotracheal tubes. The less preferred cuff utilizes a relatively inflexible, or low compliant, material. When deflated, it has a low residual volume. The "low-volume" cuff requires a large internal inflating pressure in order to expand its walls to make contact with the trachea. Any inflation beyond the point of this minimal contact generally results in the exertion of a very large pressure against the tracheal wall. This large lateral wall pressure can so seriously damage the trachea that the patient's death may in fact result.
The second type of endotracheal tube uses a cuff having a highly compliant wall and, when deflated, a large residual volume. One method of forming the high volume cuff involves placing a low volume cuff formed of polyvinylchloride in boiling water. Overinflating the cuff stretches the low volume cuff to form the large cuff.
Excessive inflation of the high volume endotracheal tube in actual use results in the exertion of less lateral wall pressure against the trachea than the low volume type. Nonetheless, the high volume endotracheal tube can still severely damage the patient's trachea. And, since it contacts a larger area of the trachea, its deleterious effects damage more of the trachea than the lower volume models.
Moreover, the deflation of the high volume cuff produces folds in the cuff's wall, a phenomenon referred to as "pruning". When expanding within the trachea, the high volume cuff does not normally expand to its full dimension. If it did, it would then exhibit the same deleterious effects as the low volume cuff. However, expansion to less than its full volume results in the cuff's wall retaining at least some of its folds. These folds, however, provide channels for the aspiration fluids to pass into the patient's lungs. The wrinkles also leave deep grooves in the trachea's mucosal lining.
Generally, the cuffs of the endotracheal tubes have a polyvinylchloride composition which, with its rough surface, can irritate the patient's trachea. Furthermore, the polyvinylchloride includes plasticizers which can leach and induce toxic reactions.
Other cuffs on endotracheal tubes have a latex or a silicone rubber composition. Both of these represent thermoset materials which require dipping and lengthy curing. As a consequence, both have nonuniform surfaces which can also irritate the trachea. Moreover, they can have pinholes and generally display a high rejection of the final product. Moreover, as thermoset plastics, a rejected item cannot undergo salvage and reuse.
Further, each product must undergo thorough testing to assure that it meets the necessary standards. Even with a perfect product, the rubber materials display substantial gas permeability. As a consequence, the internal pressure of the cuff can increase or decrease depending upon the surrounding atmosphere in the trachea. Either event would, of course, produce deleterious consequences. Increasing the pressure further irritates the trachea; decreasing the pressure may allow for the leakage of aspiration fluids around the cuff.
U.S. Pat. No. 4,154,244, to L. F. Becker et al. suggests using a block copolymer similar to the Shell materials above for both the tube's shank and cuff. Specifically, Becker et al. utilize different formulations of the styrene-ethylene-butylene-styrene block copolymer to achieve the different properties of the inflatable cuff as opposed to the substantially rigid shank. Moreover, the material has a rough surface which can irritate the patient's trachea. Also, the composition does not extrude into extremely thin sections which would provide the cuff with the desired high degree of pliancy. Accordingly, the search for improved materials for endotracheal tubes continues.